A variety of applications seek to control and/or manipulate the propagation of electromagnetic radiation and many applications are specifically directed to the manipulation and control of light propagation. For instance, lasers, optical delay components, sensors, non-linear optical devices and others manipulate light for use in many different applications.
One approach to manipulating light involves the use of crystalline structures and, more particularly, photonic crystals. Photonic crystals are structures typically implemented with dielectric type materials and are used to manipulate the propagation of light in certain applications. In many applications, photonic crystals are implemented with artificial multidimensional periodic structures having periodic variations in dielectric constant, with a period of the order of optical wavelength. These periodic structures tend to prohibit light from propagating under certain conditions. Photonic crystals can be implemented for defining a path for light that can bend sharply with low loss, for facilitating the localization of light and/or for defining small optical cavities for laser applications.
Achieving slow (or small) group velocity is useful in a variety of applications, ranging from optical delay components and low-threshold lasers, to sensors and the study of nonlinear optics phenomena.
Photonic crystals have been employed for achieving slow group velocities of light at electromagnetic band edges of the crystals. However this approach can be used to achieve slow group velocity for a relatively narrow range of wave vectors in a particular direction. This approach causes a generally large variation of group velocity with a wave vector (i.e., group velocity dispersion) and as a result leads to distortion in the shape of an optical pulse propagating through the structure.
Coupled resonator optical waveguides (CROWs) are structures that have been proposed for reducing group velocity. See, e.g., Yariv et al., Coupled-resonator optical waveguide: a proposal and analysis, Optics Letters Vol. 24, No. 11 (Jun. 1, 1999). In this case, adjacent defect cavities exhibit electromagnetic fields that couple with one another (due to evanescent Bloch waves), and a slow group velocity is achieved as a result of the tunneling of photons between the cavities. However, the reduction of group velocity is generally limited to a narrow region along the waveguide axis in which the cavities are coupled. As in the case of photonic crystal waveguides, the coupling into such a CROW structure is difficult, since the input (light) beam needs to be aligned in one particular direction.
These and other issues have been particularly challenging to the implementation of light in many applications, including applications involving the use of photonic crystals.